IgE plays a critical role in human immediate hypersensitivity ("allergic") disorders which affect upward of 30% of humans worldwide. The objective of this proposal is to continue to delineate the biology of the newly discovered family of secreted human IgE proteins as related to the expression of allergic disease, and to dissect the mechanisms that regulate the generation of these proteins via alternative epsilon RNA splicing. The production of specific patterns for the e mRNAs correlate with allergic disorders, and the absolute and relative expression of these e mRNAs is developmentally regulated. At least 3 of the 4 isoforms have been detected in vivo. The investigators have now shown differences in isoform in vitro and in vivo biologic functions. In support of the hypothesis is that the novel IgE protein isoforms possess distinct biologic properties that can provide for differences in human allergic disease expression. They will impact allergic disease expression by exploring and ultimately exploiting such differences. In Aim #1, they will determine differences in IgE isoform-ligand interactions that may result in altered in vivo and in vitro outcomes. They will determine (a) the ability of the different IgE isoforms to bind to key molecules (FceRI, FceRII, Galactin-3, Histamine Releasing Facto, and FcgRII/III) that mediate the effects of IgEs, (b) the nature and functions of the high molecular weight form of IgE tail piece (IgE Htp), and (c) the ability of the specific IgE isoforms to mediate basophil, mast cell and macrophage/monocyte functions related to IgE. In Aim #2, they will determine (a) the tissue and cell specificity of the factors controlling alternative e RAN splicing; and (b) the molecular mechanism involved in the observed developmental shift in e alternative splicing by using an alternative splicing substrate for production of mRNAs of e tail piece/grande vs IgE classic. In Aim#3, they will test the hypothesis that in vivo differential expression of the IgE protein isoforms contributes to disease expression by: (a) determining the absolute and relative amounts of the human IgE isoforms: 1) in specific human IgE-related diseases; ii) in response to allergen challenge; and iii) in tissues involved in allergic manifestations, and by (b) determining the ability of the IgE isoforms to mediate cutaneous, respiratory and systemic allergic reactivity in human FCeRIa transgenic mice.